For nearly a thousand yearsthat is, for as long as university students have been writing thesesthe last hurdle in the process has been an oral examination. Your thesis director and the two other readers you have chosen make up the examining committee. Often, this meeting is referred to as "defending" your thesis, although in modern times examinations are usually much more cordial and less adversarial than the word "defense" would imply.
Most students find it rewarding and intellectually exciting to spend an hour discussing their project with three people who have already expressed great interest by agreeing to be mentors. If you have been in close communication with your thesis director and committee during the research and writing stages, and if you can say with assurance that you have done your best work, there is no reason not to approach the oral examination with confidence and good cheer.
Your thesis director will be in charge of the session, and you should ask him or her what you might expect in terms of procedures and types of questions. You may be asked to begin the exam with a statement of your thesis and a brief summary of your work (if so, you will want to prepare rather than trying to "wing it"). The members of the committee will then take turns asking you questions.
Your adviser or committee may have some final changes or corrections for you to make before you print out the final copy that is going to be bound and preserved. This is, in fact, a fairly common occurrence, and you should schedule the defense early enough to allow for these final corrections.
When you have satisfied your committee, each will sign the approval page attesting that your work meets the standards of your field for an undergraduate honors thesis. (Rather than having your professors sign one approval sheet and photocopying it, you will want to create a sheet of original signatures for each copy of the thesis you need.)